Soaps and detergents are surfactants/emulsifiers. They combine with a variety of molecules and break large chunks of "dirt" into smaller ones. Because of the soap's chemistry, these smaller pieces are able to be grabbed by water molecules and lifted away. This reaction would apply to stuff like a fish's slime covering! Soap also isn't so pure, it also has various fragrances, lotions, and/or anti-bacterial agents. These extras can cause as many problems as the soap. Exactly how it's toxic, sorry - I don't know the biochemistry there! For more detailed chemical info on soaps, you may want to try googling "how does soap work". Several good sites popped up.